The general area of devices capable of assessing a heating or cooling system contains several patents of interest.
U.S. Pat. No. 3,417,616, issued Dec. 24, 1968 to W. A. Smith, pertains to such a device. However, the patented device measures the output temperature of the furnace and calculates cumulative BTUs, and therefore measures only the output energy. Thus, a drop in efficiency of the overall system would not be seen. For example, if the furnace efficiency were to drop by 10%, it would have to run 10% longer to achieve the same output BTU. It would be desirable to have a device capable of indicating and taking into account the extended run time, however the device described in U.S. Pat. No. 3,417,616 would show the same output. The patented device also may be inaccurate for two reasons:
1. In order to estimate fuel consumption, it assumes a constant value for furnace efficiency. This assumption is not necessarily valid in all cases.
2. By measuring only the outlet temperature for the heating (or cooling) plant rather than the temperature differential between supply and return, it does not account for changes in the return temperature which will arise due to planned seasonal temperature setbacks, or due to variances in heat delivered to the building for the same supply temperature. It assumes a constant return temperature. The system described in this patent would be suitable only with steam, where heat is delivered by condensation and the return temperature is constant (or irrelevant since heat is transferred by latent heat rather than sensible heat).
Another patent of interest is U.S. Pat. No. 4,234,927, issued Nov. 18, 1980 to T. C. First, which "indicates" degree days by sensing the temperature in individual apartment units. The purpose of the monitoring system described by First is not clear and no means of efficiency measurement is presented in this patent. It is not possible to measure degree days and apartment energy efficiency without measuring outside temperature, and this measurement is not suggested in the First patent. It is presumed that the purpose of the device described is for measurement of apartment energy consumption and the performance of heating system only, and not the efficiency of the individual apartments themselves.
In view of the shortcomings of the prior art as discussed above, it would be desirable to provide a device which was capable of measuring degree days directly, and comparing this to an accumulated run time for the furnace or other heating plant, since this could generate an index which would demonstrate both the changes in the building efficiency, and the efficiency of the heating system itself.
Accordingly, it is an aim of one aspect of this invention to provide a diagnostic thermostat which combines the foregoing monitoring capabilities with the normal function of a thermostat. Because the thermostat is already controlling the heating plant, part of the computation or sensing mechanism required is already present and supplied by the thermostat function.
Likewise the previously described patent to First, U.S. Pat. No. 4,234,927, and another U.S. Pat. No. 4,388,692 to Jones, the preferred embodiment of the present invention utilizes a microprocessor.
Certain further considerations need to be explained before describing the present invention in detail.
(A) Firstly, because of the transient effects due to building thermal capacitance and weather variables such as wind or solar access, any system comparing energy consumption with degree days will be inaccurate except over long time periods (i.e. the heating season) or for exceptional steady state weather conditions. Serious errors, up to 25%, can be introduced into the value of any short-term feedback mechanism that indicates energy efficiency. Since short-term performance information is of major interest in controlling system efficiency, this problem can seriously limit product usefulness and marketability.
(B) Another consideration has to do with the degree of sophistication of a system. Any system which provides information in terms of degree days, or building heat loss coefficient, will find its marketability to the general public quite limited, due to the fact that most people are not familiar with these terms. In addition, each building has different furnace and heating characteristics, and this will make the data generated difficult to understand for the average consumer, and impossible to compare with that of his next door neighbour.
Many currently available electronic thermostats suffer from what is called ergonomic complexity. The number of buttons and functions make them difficult for the average consumer to learn without extensive study. Many consumers and tradespeople have expressed frustration with current thermostat designs.
(C) Lastly, many of the currently available devices for indicating building efficiency are expensive and beyond the technical and economic means of the general consumer.
In regard to the consideration listed above under (A), in a preferred embodiment the present invention constitutes a simple means of providing reliable short-term data indicating energy efficiency. The transient effects due to building capacitance and weather are eliminated by selecting only significant data below a given outside temperature average. Further, the data are averaged over a two-week sampling period to eliminate weather transient effects. They are then normalized by comparison with a baseline performance value for the building stored in the microprocessor. The normalized data can then be presented in the form of a percent or `energy index` which can be easily understood by the consumer.
To address the complexity of user interaction described under (B), in a preferred embodiment of the present invention the thermostat itself has been designed, using a masking means, to prioritize user interaction according to the level of sophistication. A door can be closed in such a way that only certain program buttons are accessible through the opening provided. These are the buttons used to override the temperature on a temporary basis, and can be operated without any knowledge of further programming. The closed door also shows only the relevant parts of the LCD display needed for this simple level of consumer interaction. This arrangement is particularly appropriate for babysitters or house occupants who need a simple means of temporary temperature control.
With the door open, the full programming capabilities of the unit are available.
The thermostat can be designed to present the information on a large menu-driven LCD display which allows easy use without user training. Periods of high and low energy consumption can be presented as a bar graph on the display for quick reference.
A further advantage of this diagnostic thermostat/energy monitoring system is that it is low in cost and can be offered to the user at a price comparable to other thermostats now on the market.
In this way, the present invention in a preferred embodiment presents four major advantages over the current art:
1. Through careful data selection, it eliminates the effects of building capacitance and weather transients which would otherwise introduce large errors into the performance data.
2. Through the presentation of the data in the form of a normalized `energy index` on a menu-driven display, the information is readily understood by the average consumer.
3. By the design of the door, keypad and display, the preferred embodiment of the thermostat prioritizes its operations from an ergonomic point of view. This greatly simplifies the problem of user interaction.
4. The present invention is much lower in cost then other systems, being about the same price as a contemporary thermostat.